For heating in mobile devices (motor vehicles, airplanes, etc.), electrically operated ceramic heating elements with a positive temperature coefficient (PTC) are increasingly being used. The growing interest in such PTC heaters is notable, because it features self-limiting of the heater temperature based on the positive thermal coefficient and it results in self-regulation of the supplied and emitted power.
From a physical point of view, PTC heaters have temperature-dependent and voltage-dependent resistance characteristics. While the temperature-dependent characteristic is desired due to the temperature-limiting self-regulation, the voltage-dependent characteristic is frequently not desired, because it leads to a distortion factor that cannot be tolerated for sensitive fields of use, e.g., aerospace applications.
For such applications of PTC heaters, there is a need to limit the disruptive harmonic waves and thus the distortion factor to a permissible level.
In the prior art, it is known to suppress undesirably high harmonic-wave components by means of passive LC filters or active inverter circuits. In particular, in frequency-variable supply networks, such solutions, however, lead to an expensive circuit with respect to weight, volume, and cost.
Furthermore, in U.S. Pat. No. 5,751,138 a circuit for compensating harmonic-wave noise in a network voltage system is described, in which a pulse width-modulated inverter is controlled for each network phase, in order to generate a harmonic-wave compensation signal, which is coupled into an impedance coil. The impedance coil is arranged in series between a square-wave inverter and a respective phase, with the square-wave inverter generating a current that compensates in phase opposition the reactive impedance of the load in the respective phase of the current path. In this way, the square-wave inverter is decoupled from the pulse width-modulated inverter and is to be operated at a frequency that is considerably higher than the base frequency, so that just the voltage supply for this signifies an unjustifiably high expense.
Furthermore, circuits for reducing harmonic-wave components of the output signal are known from digital audio amplifiers. Such a digital, adaptive, feedback harmonic-wave compensation for digitally controlled power stages is described in US Published Application 2006/0034365 A1, in which an opposite-phase signal for compensating the harmonic waves is added to the input signal in an actively controlled way. The magnitude and phase of the harmonic waves are here determined by a digital pick-up circuit from the input variables of modulation degree, magnitude of the input signal, and time-error statistics of various power components and thus exceed the expense in terms of weight, installation space, and costs permissible for components, in particular, in aerospace applications.
The invention is based on the problem of finding a new possibility for harmonic-wave suppression for AC voltage-operated PTC heaters, in which the harmonic waves are compensated by superimposing a current generated inverted relative to the harmonic-wave component and which allow a simple and economical generation of the harmonic-wave suppression signal.